1. Field of the Invention
The present invention relates to a sheet post-processing apparatus which is capable of binding or punching holes in stacks of sheets after images are formed by an image forming machine, such as a copy machine, printer or digital printing apparatus, and the present invention also relates to image forming machines which use this type of sheet post-processing apparatus.
2. Description of the Related Arts
It is a well known concept in the art for copying machines and printers to transfer either an image from a series of sheets or electronic data to the surface of paper media, and to discharge that media in a predetermined order, one sheet at a time, from a discharge exit located on such machines. It is also a widely know concept for a finishing apparatus to automatically bind a stack of a series of related sheets for which image transfer has been completed using a stapling device or other binding material or to automatically punch holes in a stack of sheets for the ease of filing.
In systems described in U.S. Pat. No. 5,083,760 (hereinafter "U.S. '760") and U.S. Pat. No. 5,137,265 (hereinafter "U.S. '265"), sheets are sequentially ejected in a specified order from a well known type of finisher to a discharge side where the stack is bound using a stapler activated by a signal indicating that a predetermined number of a stack of sheets has been completed. In such systems as taught in U.S. '760 and U.S. '265, sheets that are discharged out of an image forming machine are temporarily stacked in a first tray for the stapler. After binding the stack using the stapler, this stack of sheets is then discharged to a second tray for stacking the stapled sheets.
A binding mechanism is taught in U.S. Pat. No. 4,720,033 (hereinafter "U.S. '033").
Japanese Laid Open Patent Tokkai Hei 6-135620 (hereinafter "JP '620") describes a system in which a series of sheets can be bound by a stapler at the same time as holes are automatically punched through the stack of sheets. It is well known in the art for a system to include a mechanism along the passage of the sheets discharged from an image forming machine to a tray to punch holes in those sheets and it is also well known to punch holes at the same time as binding a stack of sheets that have been fed sequentially for the purpose of binding.
Nonetheless, these types of image forming systems do not effectively respond to the following problems of a finisher, which result from the forming of an image either on one side or on both sides of the paper sheets.
There is a problem of images being formed by an image forming machine on one and both sides of the paper sheets and that such original documents can and often do exist in the same stack for copying. As these single and double-sided sheets are discharged from the image forming machine, the correct hole punching positions and the binding positions of a stapler become opposed resulting in an incorrect finishing of the stack.
To explain in more detail, a first sheet is processed with an image on only one of its sides, and for purposes of this explanation we shall call that first side the front side. Directly following in sequential order is another sheet with images formed on both sides, or in other words, on its front and back sides. When both sheets are discharged from the image forming machine, one sheet will be positioned to have holes punched in its right side while the other will be positioned to have holes punched in its left side.
This kind of problem occurs when single sheets are processed with an image on one and on both sides. The problem of having holes punched on both sides, namely the left and right sides, exists when using the systems in the U.S. '760, U.S. '033 and JP '620.